Interferometric spectrometer utilizing three fabry-perot etalons in series



E- MACK ETAL March 19, 1968 J, 3,373,651

INTERFEROMETRIC SPECTROMETER UTILIZING THREE FABRY-PEROT ETALONS IN SERIES 1963 8 Sheets-Sheet 1 Original Filed June 21,

INVENTORS.

JUL/AN E. MACK Dar/44,4: R Ala/V077- BY FEEDER/0Y1 .Aasu ER March 19, 1968 J. E. MACK ETAL 3,373,651

INTERFEROMETRIC SPECTROMETER UTILIZING THREE FABRY-PEROT ETALONS IN SERIES Zfiginal Filed June 21, 1963 8 Sheets-Sheet 2 March 19, 1968 J. E. MACK ETAL 3,373,651

G THREE INTERFEROMETRIC SPECTROMETER UTILIZIN FABRY-PEROT ETALONS IN SERIES lriginal Filed June 21 1963 8 Sheets-Sheet "5 75:32 Davao zmi QQN 7 1 7 5 E v E wwg flm n. UIY VEAL T 1 x m A 5K NAM g m a I M m March 1968 J. E. MACK ETAL INTERFEROMETRIC SPECTROMETER UTILIZING THR FABRY-PEROT ETALONS IN SERIES Original Filed June 21, 1963 8 Sheets-Sheet 4 Wm Kufl wk M TMMF N L F map a 1 5 1 mi. NNN w u E wm N WWW MxQQ MKQQQ w mm March 19, 1968 J. E. MACK ETAL INTERFEROMETRIC SPECTROMETER UTILIZING THR FABRY-PEROT E'I'ALONS IN SERIES Original Filed June 21, 1963 8 Sheets-Sheet 5 k M. m m flw me u/7m V 6 K NNAR nn/E A U0 JDPWM March 19, 1968 1 E. MACK ETAL 3,373,651

INTERFEROMETRIC SPECTROMETER UTILIZING THREE FABRY-PEROT ETALONS IN SERIES Original Filed June 21, 1963 8 Sheets-Sheet s 71: 16. llfK/M J62 firar 265 J 3/5 NW.

Juz/AN .5 MACK 004/6046 Ala/V077 BY FPfflER/CK 1. Poems? I 514% zwsy March 19, 1968 J. E. MACK ETAL 3,373,651

INTERFEROMETRIC SPECTROMETER UTILIZING THREE FABHY-PEROT ETALONS IN SERIES Original Filed June 21, 1963 8 Sheets-Sheet 7 MHI'Ch 1968 J. E. MACK ETAL 3,373,651

INTERFEROMETRIC SPECTROMETER UTILIZING THREE FABRY-PEROT ETALONS IN SERIES Original Filed June 21, 1965 8 Sheets-Sheet 8 T11 E JE- INVENTORS.

NEYS.

United States Patent ABSTRACT OF THE DISCLOSURE An interferometric spectrometer using three Fabry- Perot etalons in series. The spacings between the plates of the three etalons are different from one another, and are selected so that one peak of the transmittance curve of each etalon coincides in frequency with one peak of the transmittance curve of each other etalon, with all other peaks of the transmittance curves of the etalons being substantially non-coincident in frequency. The output of the spectrometer is detected by a phototube, and is recorded. The spectrometer can be scanned; that is, the transmittance curve center frequency can be varied so as to provide a spectral response curve for an input light source over a desired range of frequencies. This is done by means of an electro-pneumatic control system which varies the gas pressure between the etalon plates at a desired rate, while simultaneously precisely maintaining constant the pressure differentials between the etalons.

This application is a continuation of application S.N. 290,287, filed June 21, 1963 and now abandoned.

This invention relates to interferometric spectrometers; more particularly, it relates to interferometric spectrometers utilizing a plurality of Fabry-Perot etalons.

The Fabry-Perot etalon is a well-known instrument for use in spectrometry and other similar optical work. This instrument, in its elementary form, comprises a pair of transparent plates mounted parallel to and spaced apart from one another on a mounting block. When a beam of light is passed through the plates, a certain portion of the light is reflected back and forth between the plates and then passes through the plates where it combines with the light already transmitted through the plates without reflection or with a smaller number of reflections. This combination of light radiation creates a pattern of alternately light and dark concentric bands of illumination which is known as a Haidinger fringe pattern. From this pattern may be determined the spectral structure of the incoming light bear. That is, the intensity of components of the light having various frequencies within the spectrum of the incoming light may be determined. For example, the intensity of the light transmitted through the etalon can be plotted as a function of frequency. This intensity may be detected by the naked eye, by photographs taken of the Haidinger fringes, by use of a photoelectric detector coupled with a recording device, or by other known methods.

In many respects modern Fabry-Perot etalons are very satisfactory devices for use in spectrometry. They have relatively high resolution, luminosity, and filtrage. That is, they are highly capable of distinguishing between spectral components having frequencies close to one another, they transmit a relatively large portion of the light they receive, and a relatively large proportion of the light they transmit is useful in spectrometry. However, in modern spectrometry it is often desired to isolate and analyze only a very narrow portion of the spectrum of a light source. Since the etalon transmits spec- 3,373,651 Patented Mar. 19, 1968 tral components over a much wider range than is desired for such work, spectrometers have been developed in which a refracting prism or diffraction grating is used in series with an etalon or several dilferently-tuned etalons in order to remove some of the spectral components and thereby restrict the frequency range of the transmitted components to a relatively narrow band. A description of some of these prior devices can be found for example, in Condon and Odishaw, Handbook of Physics, pp. 6-96 through 6-108, McGraw-Hill Publishing Co., New York, 1958.

The addition of such a grating or prism in series with one or more etalons has proved undesirable since it substantially reduces the luminosity and otherwise impairs the performance of the spectrometer. In order to overcome these defects and provide a relatively high luminosity for a given resolution, it is necessary to greatly increase the size of the spectrometer. This makes the instrument far too large to be portable and makes it very expensive to build and house. In short, it has proved im practical to build a spectrometer of the type described above in which the product of its luminosity and resolution is at a satisfactorily high level.

Accordingly, it is an object of this invention to provide an interferometric spectrometer utilizing a plurality of Fabry-Perot etalons and having a high resolution together with a high luminosity, while being relatively small in size.

Another object of this invention is to provide such a spectrometer which is so small that it is easily portable and is rugged enough to be moved from place to place without losing its precision or desirable operational qualities.

A multiple-etalon spectrometer in accordance with one embodiment of the present invention uses a scanning system in which the pressure of the air between the glass plates of each etalon is varied to change the narrow band of frequencies transmitted by the spectrometer, In order to obtain substantially complete rejection of light having undesired frequencies, it is desirable to control the pressure in each etalon with a high degree of accuracy.

Accordingly, it is a further object of this invention to provide improved apparatus and methods for scanning multiple-etalon spectrometers.

Another object of this invention is to provide simple and precise apparatus and methods for pressure-scanning such spectrometers.

Still further, an object of this invention is to provide an electrical actuating system for such a pressure control system, the electrical control system being fast acting, relatively simple in construction, and highly reliable.

The drawings and descriptions that-follow describe the invention and indicate some of the ways in which it can be used so as to meet the above-stated objects. In addition, some of the advantages provided by the invention will be pointed out.

In the drawings:

FIGURE 1 is an elevation view of a portion of an interferometric spectrometer in accordance with one embodiment of the present invention;

FIGURE 2 is a schematic diagram of the optical system of the spectrometer shown in FIGURE 1;

FIGURE 3 is an elevation view of an etalon chamber of the spectrometer shown in FIGURE 1;

FIGURE 4 is a sectional view taken along lines 44 of FIGURE 3, in the direction of the arrows;

FIGURE 5 is another sectional view taken along lines 5-5 of FIGURES 3 and 4;

FIGURE 6 is an enlarged, partially cut-away portion of the view shown in FIGURE 4;

FIGURE 7 is a schematic diagram of a pressure scanning and control system in accordance with the present invention;

FIGURE 8 is a simplified schematic diagram showing the interconnections of the electrical and pressure components of the system shown in FIGURE 7 during its operation in scanning the spectrometer;

FIGURE 9 is a more detailed schematic diagram of the pressure control system when operated in the mode shown in FIGURE 8;

FIGURE 10 is a schematic diagram of the pressure control system shown in FIGURE 7 when it is operated so as to maintain at a constant value the pressure in the etalon chambers of a spectrometer;

FIGURE 11 is a schematic drawing of the pressure control system of FIGURE 7 when it is operated to evacuate the etalon chambers of a spectrometer;

FIGURE 12 is a schematic diagram of an electrical control component utilized in the pressure control system shown in FIGURE 7;

FIGURE 13a is a reproduction of a graph obtained by use of a spectrometer in accordance with the present invention;

FIGURE 13b is another reproduction of a graph obtained by use of a spectrometer in accordance with the present invention;

FIGURE 14 is a reproduction of an actual spectral transmittance recording made for various components of a spectrometer constructed and operated in accordance with the present invention;

FIGURE 15 is a schematic diagram of an alternative arrangement for measuring pressure in the etalon chamber; and

FIGURE 16 is another alternative pressure-measurement circuit similar to that shown in FIGURE 15.

General description An interferometric spectrometer in accordance with the present invention is generally indicated at 20 in FIG- URES 1 and 2. A light beam to be analyzed is directed into the spectrometer in the path and direction indicated by arrow 22. Thi light beam passes through a colored filter 24 (in applications where the use of such a filter is desirable) and a condensing lens 26, and then through a reflecting prism 28. The reflected light beam passes through an aperture in a plate 32 and on to a collimating lens 34. The collimated beam or pencil of light then passes through an interference filter 36 and then through three Fabry-Perot etalons 38, 40 and 42 arranged in series with one another. The beam transmitted by etalons 38, 40 and 42 passes through a telescope lens 44 and an aperture 46 in a plate 48 located at the focal plane of the telescope lens and on to a photomultiplier device 50. A recorder (not shown) is connected to photomultiplier 50 to provide a graphic recording of the light signals received by the photomultiplier.

As is shown in FIGURE 1, etalons 38, 40 and 42 are housed, respectively, in etalon chambers 52, 54 and 56. These chambers and the remainder of the spectrometer structure are vertically-aligned and. mounted together onto a solid bench 57 or similar support structure. Pairs of air pressure lines 58 and 60, 62 and 64, and 66 and 68 are connected, respectively, into housings 52, S4 and 56. These air pressure lines are connected to the pressure control apparatus shown in FIGURE 7 which varies the pressure in each of the chambers 52, 54 and 56 in order to scan" the spectrometer, as will be described in greater detail below. Chamber 52 is shown with its cover secured in place While chambers 54 and 56 are shown with their covers removed. Chamber 54 is also shown with its etalon removed whereas chamber 56 is shown with its etalon 42 in position.

The optical system and structure of the spectrometer The holders for the filter 24, the lens 26 and the reflecting prism 28 are mounted on a table 70 (see FIG- URE 1) which is rotatable about the vertical axis of the spectrometer and has detents to hold it in position at intervals of 30 degrees. Rotatable table 70 provides a convenient means for admitting the incoming light beam from all horizontal directions. The filter 24, which may be any of a number of known chemical-type absorbing filters, is not essential to the operation of the spectrometer. It may be used where it is desired to provide preliminary filtering of the incoming light beam. Such filtering might be provided, alternatively, by a prism of low resolution or by a narrow-band detector. Similarly, the reflecting prism 28 is not essential and may be omitted if the incoming beam is introduced vertically.

The incoming beam is passed through aperture 30 in plate 32 in order that lens 34 will produce a parallel beam of light to be transmitted. Aperture 46 in plate 48 is conjugate with aperture 30 and serves to reduce the effect of scattering, fluorescence, and asymmetric reflections distorting the beam as it passes between the two apertures.

This aperture system has the effect of selecting the Haidinger fringe nearest the vertical axis of the spectrometer as the one transmitted. The advantage of this arrangement is that its geometry is much simpler than the geometry of previous instruments in which the Haidinger fringes away from the longitudinal axis were transmitted. Focusing is accomplished by screw adjustment of a table designed to hold plate 32. An arrangement is provided for laterally adjusting the second aperture 46 so as to optimize the illumination of the photomultiplier 50 The collimating lens 34 and the telescope lens 44 may be identical. They need be corrected only for abberations near their central axes. The telescope lens 44 is mounted in a frame 72 which can be laterally adjusted by means of thumb screws 74. A pair of lens tubes 76 and 78 are used to transmit the light beam between the apertures 30 and 46 and the lenses 34 and 44. Access doors 80 and 82 are provided, respectively, in lens tubes 76 and 78. A I

mirror can be inserted through either of doors 80 and 82 and can be mounted at a 45 degree angle in a holding block located inside the lens tube. By the use of such mirrors it is possible to look at the spectrometer components located above or below the doors 80 and 82.

The interference filter 36 is mounted in the spectrometer 20 on a tiltable holder which allows the filter to be rotated about an axis perpendicular to the plane of the drawing as indicated by the arrows in FIGURE 2. This arrangement provides for adjustment of the filter to give it peak transmittance at a desired frequency. The filter is tiltable to a maximum angle of 15 degrees with respect to horizontal.

The interference filter 36 itself is well-known in the prior art and preferably is a Fabry-Perot type comprising an optical glass plate with a dielectric multilayer coating on its surface. For example, the interference filter comprises a glass plate with 9 layers of dielectric coating materials, a spacer and 9 more layers of dielectric materials deposited on the surface of the plate. Alternate ones of the 9 inner and the 9 outer layers have a relatively high index of refraction while the layers sandwiched be tween these alternate layers have a relatively low index of refraction. The substances usually used to form these layers are zinc sulfide and cryolite. The incoming light beams are reflected back and forth between layers in accordance with the Fabry-Perot principles to fiber out spectral components within a fixed frequency band. These interference filters have a high transmittance and low absorption, that is, they transmit most of the light they receive (within their pass band) and absorb very little of this light. This is in direct contrast to retracting prisms or diffraction gratings which waste a large percentage of the available light and therefore reduce the luminosity of a system in which they are used.

Each of the air-tight etalon chambers 52, 54 and 56 is specially designed to hold an etalon assembly with a spacing between the top of the etalon and the top of the chamber sufiicient to permit a 45 degree mirror to be inserted into the spacing so that one may observe the beam being transmitted through the optical system preceding each etalon by merely opening the etalon chamber door and inserting the mirror. The construction of chambers 52, 54 and 56 will be described in greater detail below.

The structure of etalons 38, 40 and 42 is well-known in the art. Each etalon comprises a pair of parallel glass plates which are mounted in a support structure made entirely of Invar. The spacing between the parallel plates is maintained by a set of three springs which force the plates against a stable spacer. Alternatively, any of several kinds of variable spacers might be used. The plates are given a slight wedge shape (shown exaggerated in FIGURE 2 for the purpose of clarity) in order to deflect the beams reflected from the backs of the plates away from the aperture 46 so that these reflections .are not transmitted to the photomultiplier and recording system. The outside surface of each plate is shaped to form an angle of 0.01 radian with respect to horizontal to produce this wedge shape. Each of the plates is coated with coatings similar to that used on the interference filter 36 and described above, but without the spacer. That is, each plate has, for example, seven coats comprising alternate layers of zinc sulfide and cryolite, with the first and last layers being zinc sulfide. These layers give the etalon a very high transmittance and low absorption.

It is important that the etalon plates be made flat to a high degree of precision. These plates are made so smooth that the difference between the maximum and the minimum spacing between the parallel plates is not more than 4 of the wave length of green light.

The spectrometer operates as follows. The incoming light beam first passes through the filter 24 which filters out from the beam all spectral components except those having frequencies within a relatively broad range. The light passing through filter 24 is then restricted by aperture and is formed into a parallel beam as described above. Next, this beam is passed through the interference filter 36 which removesall of the spectral components except those falling within a fixed frequency range which is substantially narrower than that passed by the filter 24.

Thus, when the parallel beam of light reaches the first etalon 38 in the spectrometer 20, all of its spectral components outside of a relatively narrow range of frequencies have been filtered out by filter 24 and interference filter 36.

Each of the etalons 38, 40 and 42 transmits the light beam it receives according to the Airy function given below:

R is the reflectance of the coatings on the etalon plates;

1 is the distance between the etalon plates, which are assumed to be parallel and perfectly flat;

n is the refractive index of the medium between the plates;

0' is the wavenumber of the incident light;

0 is the angle of incidence of the incident light, and

A is the intensity of the light transmitted by the etalon.

Since in this instrument only the fringe nearest the longitudinal axis of the spectrometer is examined, cos 0 is approximately 1 and Q(n; 0) may be replaced by the nominal free spectral range Q, where:

The spectral components transmitted by the filters 24 and 36 to the etalon system usually cover a range including a large number of orders or peaks of the Airy function. Since it is often desired in instruments such as this to isolate a single peak or order from this group of peaks, the object of the multiple etalon system is to suppress the parasitic peaks other than the desired peak. The ability of a single etalon to distinguish between closely adjacent spectral components is called its resolving power. The theoretical resolving power or resolution of a single etalon R is defined as being the ratio of the wave number of the light being examined to the resolving limit of the etalon. That is:

The resolving limit of an etalon is the total width of a peak in the Airy function at half of its height. For an etalon having ideally flat plates, it can be shown that the theoretical resolution for the etalon is a function only of the reflectance R or its plates and the wave number for an etalon having given dimensions. Since, with modern multilayer dielectric coatings used on etalon plates, the reflectance of the plates can be made quite high (e.g. 99%). The actual resolution of an etalon is a function primarily of the flatness of the plates. Since the plates can be made flat to a high degree of accuracy, the resolution of the etalon can be quite good.

The theoretical luminosity L of a single etalon is a measure of the flux or energy transmitted 'by the etalon from a source of unit luminance. The luminosity-L for a single etalon is:

S is the area of the etalon plate;

1 a transmittance of the etalon plates; and

Q(=1rB is the solid angle of the bundle of light rays illuminating the etalon.

There is a complicated interdependence 'between resolution and luminosity so that a compromise must be reached between the two in designing an etalon. However, a relatively high luminosity can be obtained without reducing the resolution seriously. Very satisfactory luminosity can be obtained with a reduction of the resolution of the etalon to 0.7 times the theoretical resolution.

The filtrage of an etalon is a measure of its effectiveness in isolating a spectral element from a continuous light source and is defined as the ratio of the useful energy, U (that is, the energy transmitted within the width of a peak of the Airy function at half of its height), to the total energy, E received at the output of the etalon. The filtrage for a single etalon is given by the following T(o') is the instrumental function of the auxiliary monochromator (filter system) characterizing the spectral quality of the light it transmits from a white source. If T(o') is a rectangular function, then F=0.5.

From the foregoing it is seen that a single etalon has high resolution, high luminosity and high filtrage. Moreover, it can be shown that when several etalons are placed in series as in the spectrometer of the present invention, it can be shown that the resolution of this system is actually greater than that of a single etalon and can be as high as twice the resolution of a single etalon. Similarly, it can be shown that the theoretical luminosity of a train of etalons is equal to the luminosity of a single 7 etalon. However, in practice, the luminosity of the multiple etalon is limited by the flatness of the plates in each etalon. Since modern techniques are used to make the plates of etalons 38, 40 and 42 flat within very close tolerances, the luminosity of these multiple etalons can be maintained at a relatively high value. Furthermore, by designing the etalon system properly, the filtrage of the multiple etalon system can be maintained at a satisfactorily high level.

From the foregoing, it is clear that the resolution of a multiple etalon system is actually better than that of a single etalon while the luminosity is only slightly impaired and the filtrage is approximately equivalent to that of a single etalon.

As was mentioned above, when a grating or retracting prism is used as an auxiliary filter with a series of etalons, the luminosity of the spectrometer is impaired. That is, in order to maintain a given resolution, all prisms or grating of a size practical for use in the spectrometer substantially reduce its luminosity. For example, a diffraction grating capable of providing a spectrometer with a resolution-luminosity product equal to that of the present spectrometer would have to be of the order of 1 square meter in size. Such a grating would be prohibitively large and expensive, and otherwise would be impractical to use.

This difficulty is avoided in the spectrometer of the present invention by using the interference filter 36 in place of a grating or prism. As a result, this spectrometer has a high luminosity-resolution product while being small in size. For example, a unit like that shown in FIGURE 1 is only 22 cm. by 22 cm. by 150 cm., as compared with prior devices which occupy whole rooms and still have a lower luminosity-resolution product. Thus, this spectrometer is portable and can be taken to vantage points such as mountain tops for astronomical observations. Further, this spectrometer can be used to analyze light beams having a very low intensity, and, even when the intensity of the light beam is sufficiently high, the graphs produced by such a spectrometer will be strong and clear in detail.

The method of isolating the peak or order desired to be recorded comprises setting the spacings l l 1 (see FIGURE 2) of the plates of etalons 38, 40 and 42, respectively, at different values. By reference back to the Airy function given above, it can be seen that with differing values for these spacings, the Airy peaks or orders will occur at different frequencies for each etalon. The ratios of the spacings 1 21 1 are chosen to approach the ideal situation in which only a single peak is passed by the etalon train. Since the filter 24 and interference filter 36 limit the range of frequencies passed to the etalons, the only peak passed is the single selected. peak.

FIGURE 14 is a reproduction of a recording of the intensity of the light transmitted from a white light source (having a continuous spectrum) through each of the separate major components of the spectrometer 20 and through various series combinations of these components, with the interference filter 36, etalon plates and spacing ratio 11:12:13, chosen to pass a single peak or order randomly chosen to have a frequency of yellow light near the sodium resonance doublet. These intensities were recorded as a function of frequency, which is expressed in K-aysers (K). The Kayser is the reciprocal of the vacuum wave length and has the unit (cm.) The frequency range selected for this recording was 100' orders or peaks of the Airy function for each etalon. The peak chosen to be passed is located at the frequency point on the graph.

Curve 0 of FIGURE 14 represents the light transmitted by the interference filter 36 alone. This filter had a half-intensity-width (6 11) of 30K=11 angstroms (A.). Its response is seen to peak at the 0 frequency point and drop off to a low value after about 20 orders.

Curves 1, 2 and 3 are, respectively, the response curves for the three etalons 38, 40 and 42, each taken alone, without auxiliary filtering. The spacings used were: l =3.O00 -mm., l =2.79O mm., and l =1.769 mm. The spacing ratio was 10019315897 (approximately 100293: 59). It can be seen that the peaks or orders of curves 1, 2 and 3 do not coincide in frequency.

Curve 1+2 is the response of etalons 38 and 40 combined (without interference filter); curve 1+3 the response of etalons 38 and 42; curve l+2+3 the response of etalons 38, 40 and 42 in series; and curve 0+l+2+3 is the response of all three etalons in series with the interference filter 36. From these curves it can be seen how the etalon combinations remove all spectral components except some small ghosts located far from the frequency desired to be passed. These ghosts are removed by the interference filter 36 so that only a single order remains in the final response curve 0+1+2+3.

As explained above, it is often desired to vary the frequency of the order or peak isolated by the spectrometer 20. The performance of this function is known as sweeping or scanning the spectrometer.

In the spectrometer 20 this scanning function is performed by varying the index of refraction n of the gas between the plates of each etalon 38, 40 and 42. In accordance with the present invention, the index n in one etalon is used as a reference and the indices :1 and 11 of the remaining etalons are set at values different from n by pre-deterrnined amounts. Then, n is varied and 11 and 11 are made to follow :1 while maintaining constant the pre-determined amount of difference. In the preferred sweeping arrangement, the indices n n and 11 are varied by changing the air pressure in the chambers 52, 54 and 56 enclosing the etalons 38, 40 and 42. The index 21 in etalon 38 is used as the reference or master index. Thus, the pressure of chamber 52 is used as the master pressure and is initially set at a pre-determined value P The pressures in chambers 54 and 56 are set at certain values P and P different from P in the initial tuning of the spectrometer. Then, the pressure in each chamber is varied in a manner such that the pressure differential between chambers 54 and 56 and the chamber 52 is maintained constant. In this manner, the initial precise tuning of the spectrometer is maintained at its optimum throughout the scanning process.

The graphs shown in FIGURES 13a and 13b are reproductions of recordings actually obtained from a spectrometer like that shown in FIGURES 1 and 2 by scanning it in the manner outlined above.

The recording in FIGURE 13a was obtained when the spectrometer was tuned to focus on the green light having a wave length around 5461 A. produced by a commercial mercury germicidal lamp. In the recording intensity of the light was plotted against frequency as an abscissa, in Kaysers. The short vertical lines intercepting the abscissa line indicate points where it was predicted that components would be found. The actual curve shows these components with a high degree of fidelity.

The spacing ratio l zl zl used to produce FIGURE 13a was 100193259. The resolution R predicted by computations was 5x10 and that actually obtained was about 4.5 X10 thus showing a close correspondence between the theoretical and actual operation of the spectrometer.

The recording in FIGURE 13b has ordinates and abscissa like those of FIGURE 1312. It shows the performance of the present spectrometer in absorption spectrometry. The curve was made by directing a beam of white light into the spectrometer (producing the response in intervals a, c and e of the curve) and then covering the source with a black card, giving the response in interval b to provide a reference response. Then the card was removed for interval c and it was replaced by an oven containing sodium vapor at about C. for the interval d. In this manner the interval a indicates the spec- 9 trometers response to the absorption of the resonance line D; of sodium vapor. The bottom of this response curve was only 4% above the theoretical low point indicated by the minimum response to the black card. This response is considered to be extremely good and is considerably better than usually is obtained with devices of this type.

A problem occurs in aligning the etalons with respect to one another because there are reflections of beams back and forth between etalons. These reflections are usually undesirable since they tend to reduce the filtrage of the spectrometer. In accordance with the present invention, these reflections may be directed away from the aperture 46 by tilting the middle etalon 40 so that it is slightly out of alignment with the other etalons.

Also in accordance with the present invention, these reflections between etalons, considered to be harmful, are used to advantage in aligning the etalons when they are initially mounted in their housings. First, the uppermost etalon 38 is aligned with the uppermost aperture 30 by the known process of auto collimation. Next, with a light source being directed into the spectrometer, the second etalon 40 is aligned with etalon 38 by moving it until the beams reflected between the etalons and shining on aperture plate 48 converge to a small spot. The third etalon 42 may be aligned with the second in the same manner. This method provides for highly accurate and simple alignment of the etalons. If there is not enough transmitted light to allow this method to be used to align the third etalon 42, the light source can be placed below aperture 46 and, with the help of a 45 degree mirror placed above the second etalon 40, the beams reflected between etalons 40 and 42 can be narrowed to a small spot to align etalon 42.

The construction of the etalon housings 52, 54 and 56 is an important feature of the present invention. Since these chambers are substantially identical to one another, only one chamber, chamber 54 is shown in FIGURES 3 through 6.

Chamber 54 comprises a box-like aluminum housing 100 having a pair of opposed window openings, generally indicated at 102 and 104, and a cover plate 106. Cover plate 106 is easily removed or replaced to faclitate entry into the chamber, and is sealed in an air-tight manner to housing 100 by means of a locking arrangement which includes a pair of notched members 108 and 110 which are secured to opposite sides of housing 100-. Members 108 and 110 may be cast integrally with housing 100. A handle 112 is rotatably mounted on the front surface of cover plate 106, and a locking bar 114 is secured to it. Rotation of handle 112 moves bar 114 into engagement with the notched portions of members 108 and 110. The inner surface of the notch in each member is canted so as to wedge the bar between it and the cover plate so as to lock the cover 106 tightly onto the housing 100. By rotating handle 112 in the direction of the arrows as indicated in FIGURE 3, the bar 114 becomes disengaged from members 108 and 110 and the cover 106 is free to be removed from the housing. A pair of stoppins 116 and 118 prevent the bar 114 from being turned too far while locking or unlocking the cover plate 106 in position. An O-ring 120 (also see FIGURE 1) is used to form an air-tight seal between cover plate 106 and housing 100.

Chamber 54 is adapted to be secured to the next etalon chamber (which, in this case, is chamber 56) by means of screw-threads 122 on a flanged portion of a window and etalon mounting member 124 which is fitted into the bottom opening 104 of housing 100. Referring especially to FIGURE 6, the top portion (indicated in dashed lines in FIGURE 6) of the next Succeeding chamber 56 is secured to chamber 54 by means of a threaded ring 126 which engages threads 122 to hold the flanged upper portion of chamber 56 tightly against the lower surface of chamber 52. O-rings 132 and 134,

10 and make the seal between the two chambers airtight.

A glass window pane 136 is mounted in holder 124. This pane 136 is made of optical glass and its surfaces are parallel to within 0.5 micron. In accordance with the present invention, it is mounted in member 124 in a position inclined with respect to horizontal at an angle of about 0.01 radian (this inclination is exaggerated in the drawings in order to show it clearly). This inclination casts any light beams that might be reflected by the windows away from the aperture 46 so that they do not interfere with the beams desired to be transmitted. A threaded ring 138 (see FIGURE 6) engages similar threads in mounting member 124 to secure window pane 136 in the member 124. An O-ring 140 provides an air-tight seal between member 124 and the window pane.

As is shown in FIGURE 5, the upper surface of window holding member 124 forms a point-line-plane mounting block which is specially adapted to accurately mount an etalon in the chamber.

Chamber 52 has an upper window (not shown) in addition to its lower window 136 so that it is sealed shut and is air-tight.

The chamber construction described above is advan tageous in that it allows the chambers to be connected together rapidly, easily, and with the high degree of accuracy required by a precision instrument of this nature. Furthermore, the cover plate locking mechanism is fastacting so that ready access to the etalons may be had for adjustment purposes, and yet fastens the cover plate securely to the housing to maintain the chamber airtight.

The scanning system The schematic diagram in FIGURE 7 is a schematic diagram of a system for controlling theair pressure in the etalon chambers 52, 54 and 56. The major components of this system are shown in dashed outline and include the spectrometer unit 20, a pressure measurement unit connected to chambers 52 and 54 for measuring the difference between the pressures P and P in chambers 52 and 54. An electronic-presure control unit 152 is electrically connected to the measurement unit and receives an electrical measurement signal proportional to the magnitude and phase of the pressure difference between chambers 52 and 54. The pressure control components of unit 52 are connected by air lines 58, 60, 62 and 64 to chambers 52 and 54. The control system is completed by a manual pressure control unit 154 and an automatic reference-pressure control unit 156.

Electronic-pressure control unit 152 contains an electrical selector switch generally indicated at 158 which is operated to control the operation of the system.

When the switch 158 is turned to the STEADY position, the electrical and pressure components are interconnected so as to automatically hold the pressure in chambers 52 and 54- at a steady value. The interconnections between these components when switch 158 is turned to the STEADY position is shown in FIG- URE 10.

When switch 158 is turned to the SWEEP position, the system automatically varies the pressures P and P with a constant pressure differential between them. The electrical and pressure components active in the system during the SWEEP mode of operation are shown in FIGURES 8 and 9.

When switch 158 is turned to the EVACUATE position, the system automatically evacuates chambers 52 and 54 and prepares them for the start of another sweep or scan of the spectrometer. The electrical and pressure connections made during this EVACUATE mode are shown in FIGURE 11.

When switch 158 is turned to the MANUAL position, the components of manual pressure control unit 154 are activated to enable the operator to manually vary the pressure in chambers 52, 54 and 56. Turning switch 158 to this MANUAL position energizes normally-closed solenoid valve 17 by means of a 24 volt DC source as shown in the drawing. The opening of valve 170 connects handoperated throttling valves 172, 174 and 176 to a vacuum pump or some other low-pressure air source. Valves 172, 174 and 176 are connected, respectively, to chambers 52, 54 and 56 to reduce the pressure in those chambers as desired. Another normally-closed solenoid valve 178 also is opened when switch 158 is turned to the MANUAL position. Solenoid valve 178 connects manually-operated throttling valves 180, 182 and 184 to a pump or other source of high pressure air. Valves 180, 182 and 184 are connected, respectively, to chambers 52, 54 and 56 and may be manually operated to increase the pressure in these chambers as desired.

It should be understood that the equipment shown for automatic use in the SWEEP, STEADY and EVAC- UAT modes of operation is shown only for controlling the pressure in chambers 52 and 54. Another identical set of components must be provided to control the pressure in chamber 56.

The pressure measurement unit 150' measures the difference in air pressure between chambers 52 and 54 and sends out an electrical signal whose polarity depends upon the direction of deviation of the pressure differential from a predetermined value. The measurement device includes a mercury manometer tube 136 with a ferrite ball 188 floating on a surface of the mercury column in the manometer tube. A differential transformer 190 is wound around the manometer tube. The windings of transformer 190 are wound on a spool (not shown) which can be moved up and down on the manometer tube. The secondary Winding 192 of transformer 190 is wound on the center of the spool with its primary windings 194 and 196 wound on either side of secondary Winding 192, as is indicated schematically in FIGURE 8. When the ferrite sphere is moved either above or below the center of the differential transformer coil structure, a signal will be developed by secondary winding 190. The primary windings 194 and 196 are energized by a 115 volt A.C. source. The voltage developed by the secondary windings 1911 will be either 90 degrees leading or 90 degrees lagging with respect to the primary voltage, depending upon whether the ferrite ball is positioned above or below the center of the differential transformer coil structure.

Photocells 198 and 20% are positioned above and below the differential transformer and are positioned, respectively, opposite lamps 202 and 204. This photocell and lamp arrangement serves as a safety device for the measurement unit in that if the ferrite ball moves above photocell 198 or below photocell 200, an electrical signal will be transmitted over either line 206 or line 208. These signals are used to institute corrections in the pressures in chambers 52 and 54 and return the ferrite ball to the center of the differential transformer structure.

Mercury manometer and differential transformer arrangements somewhat similar to that described above have been proposed in the past. However, these devices use a cylindrical metal slug in place of the ferrite sphere 188. These slugs, because of the large surface over which they contact the sides of the glass manometer tube, tend to stick and special vibratory means often must be provided in order to prevent them from sticking in the manometer tube. The ferrite ball used in the present invention eliminates this difficulty and makes it unnecessary to provide such vibratory means. It moves smoothly and freely up and down in the glass manometer column.

FIGURE 8, a simplified schematic diagram, and FIG- URE 9 a more detailed diagram, both show the electrical pressure connections of the pressure control system when it is operated in its sweep mode by turning the switch 158 to its SWEEP" position. As was explained above, in scansures P and P in chambers 52 and 54 linearly with respect to time while maintaining a fixed differential pressure DP between P and P A single source of high-pressure air is used to supply both chambers 52 and 54. Air from the high-pressure supply flows into chamber 52 through a needle valve 210 and into chamber 54 through a pressure-actuated needle or dome valve generally indicated at 212. The pressure supplied by the high pressure source is relatively high compared with the pressures to be attained in chambers 52 and 54 so that the changes in these pressures will not have any significant effect on the rate of air flow through needle valves 210 and 212. Under these conditions, air flows through needle valves 210 and 212 at supersonic speeds. This flow is substantially constant with respect to time and causes the pressures in chambers 52 and 54 to increase linearly.

Before switching into the SWEEP mode of operation, the desired pressure differential DP between P and P is set up between the chambers 52 and 54 when the spectrometer is initially tuned by use of the manual pressure control system. When the desired tuning has been obtained, the differential transformer is moved to a position surrounding the ferrite ball 188 so that secondary winding 192 produces no output signal. Then the switch 158 is switched to its SWEEP position and normallyclosed solenoid valves 210 and 214 (see FIGURE 9) are opened to allow high-pressure air to flow through needle valves 210 and 212 to increase the pressure in both chambers 52 and 54. If the rate of pressure increase through needle valve 212 is different from that through needle valve 210, the pressure control system will automatically adjust needle valve 212 to equalize the rate of pressure increase through the two valves. This adjustment is accomplished as follows.

If, after a short time, P is either too low or too high so that the difference between P and P is no longer equal to the initial value DP, an error signal will be developed by secondary winding 192 of differential transformer 190. This error signal will be transmitted to an amplifier 216 and then to a phase switch 218 which will send an electrical signal over output line 220 if P is too low, or a signal over output line 222 is P is too high. If a signal is sent out on line 222 indicating that P is too high, this electrical signal opens normally-closed solenoid valves 224 and 226 which are connected to a common source of low pressure or vacuum. Valves 224 and 226 remain open until the pressure P in chamber 54 has been reduced to a value such that an error signal is no longer produced by the differential transformer 190. During this same time interval, valve 224 connects the vacuum source to the dome portion 228 of valve 212. Dome portion 228 includes a diaphragm 230 and a spring 232 which are coupled to the piston 234 of needle valve 212. The opening of valve 224 therefore reduces the pressure on the right side of diaphragm 230 and spring 232 moves piston 234 a small distance to the right to close the orifice of needle valve 212 by a small amount. This adjusts the rate of pressure increase through needle valve 212 and tends to make this rate equal to that through valve 210.

When the pressure in chamber 54 is too low, two normally-closed solenoid valves 236 and 238 are opened by an electrical signal which is sent out over line 220. The opening of valve 23-8 connects the high-pressure line directly to chamber 54 so as to by-pass needle valve 212 and increase pressure P quickly. Simultaneously, solenoid valve 236 connects the same high-pressure source to the dome 228 of valve 212. This causes plunger 234 to move to the left, opening the orifice of valve 212 by a small amount so that the rate of pressure increase through valve 212 is increased.

After several cycles of adjustment in the manner described above, the rate of air flow and pressure increase through the needle valve 212 is essentially the same as that through 210 and the only further corrections made during the scanning process are of a random nature.

An important advantage of this control system is that it is automatic and highly accurate. This accuracy is desired since small deviations from the desired DP can derogate substantially from the performance of the spectrometer.

Referring now to FIGURE 9, manually operated throttling valves 240, 242, 244, 246, 248 and 250 are connected in series, respectively, with valves 224, 236, 214, 210, 238 and 226 to provide for fine adjustment of the fiow through the latter valves. In particular, the throttling valve 246 provides for fine adjustment of the flow rate through valve 210 and, therefore, provides for fine adjustment of the SWEEP rate of the spectrometer.

- Phase switch 218 actually has two separate switching channels 252 and 254, each of which is shown in greater detail in FIGURE 12 and each of which is connected through a fine adjustment potentiometer 253 or 255 to amplifier 216 to receive its error signal output. Channel 252 sends out a signal over line 257 and channel 254 sends a signal over line 259 if the error signal indicates that P is too high. Channel 252 sends out a signal over line 261 and channel 254 a signal over line 263 if P is too low.

An amplifier 256 is connected to line 206 and receives and amplifies signals from safety photocell 198. Its output is connected to line 261 so that if photocell 198 should be actuated by the movement of ferrite ball 188 above differential transfonmer 190, an amplified signal would be sent to open the normally-closed solenoid valve 238 and quickly raise the pressure of chamber 54.

Similarly, an amplifier 258 is connected to line 208 to receive and amplify similar signals produced by safety photocell 200 when the ferrite ball moves below differential transformer 190. The output of amplifier 258 is connected to line 257 and opens valve 226 to quickly reduce the pressure in chamber 54.

When the switch 158 is turned to the STEADY mode, the electrical and pressure components are connected together as shown in FIGURE 10. In this arrangement, the pressure in chambers '52 and 54 is maintained constant. Chamber 52 is sealed shut by valve 210, which now is closed. Chamber 54 is connected to solenoid valves 226 and 238 which are connected, respectively, to the vacuum and pressure sources. If the difference in pressure DP between chambers 52 and 54 varies during this steady mode of operation, a signal is sent out by differential transformer 190 through amplifier 216 and one channel of phase switch 218. If the pressure P in chamber 54 is too low, valve 238 will be opened ina manner like that described above in order to increase the pressure P Similarly, if the pressure P is too high, valve 226 will be opened to evacuate some air from chamber 54 and reduce its pressure to the desired value. Safety photocells 198 and 200 are included in the circuit and they perform the functions described above.

When switch 158 is turned to the EVACUATE position, the connections shown in FIGURE 11 are made in order to evacuate the chambers 52 and 54 and return them to the initial position preparatory to scanning the spectrometer again. Normally-closed solenoid valve 260 is energized during this mode of operation. Valve 260 connects chamber 52 through a throttling adjustment valve 262 to a vacuum supply. During the evacuation of chamber 52, chamber 54 also is evacuated through valve 226. The remainder of the electrical and pressure components are connected as in FIGURE and operate to maintain DP constant during the evacuation.

A notable feature of this arrangement is that during the EVACUATION and STEADY modes of operation the pressure in the dome portion 228 of valve 212 is maintained constant. Valves 224 and 226 are closed and prevent air in the dome 228 from escaping. An advantage of this arrangement is that when the next sweeping action is started, the pressure in dome 228 will be set at a value which is appropriately correct so that only relatively small corrections will need to be made by the differential transformer and manometer unit.

One channel of the phase switch 218 is shown schematically in FIGURE 12. It comprises an isolation and impedance-matching transformer 300 having applied to its primary winding the error signal generated by differential transformer secondary 192 and amplified by amplifier 216. The error signal is transmitted from the secondary winding of transformer 300 by conductors 302 and 304 to a silicon-controlled rectifier gating circuit 306. This gating circuit produces pulses at output lead 307 which are applied to the gate leads 308 and 310 of a pair of silicon-controlled rectifiers (SCRs) 312 and 314.

Gating circuit 306 is of a known type and will not be described in detail. It produces gating pulses when the polarity of line 302 is negative with respect to line 304.

Leads 261 and 263 (see FIGURES 7 through 9) are connected to the anode of SCR 312 and are connected, respectively, to the solenoids of valves 238 and 236. Similarly, leads 257 and 259 are connected to the anode of SCR 314 and are connected, respectively, to the solenoids of valves 226 and 224. Two free-wheeling diodes 316 and 3-18 are connected in series with leads 261 and 257, respectively, in order to prevent the solenoid valves from chattering.

A transformer 320 has a center-tapped secondary winding 322 which is connected between lines 261 and 257. Transformer 320 is energized from a standard volt A.C. supply.

As was explained above, the polarity of the error signal depends upon the position of the ferrite ball 188 with respect to the differential transformer 190. Thus, the error voltage applied to transformer 300 will be either in phase or degrees out of phase with respect to the 115 volt A.C. supplied to transformer 320. Thus, a gating pulse produced by gating circuit 306, when line 302 goes negative with respect to line 304, will turn on either SCR 312 or SCR 314, depending upon whether the error signal is in or out of phase with the 115 volt A.C. supply. In this manner, electrical pulses are sent out over either lines 261 and 263 or 257 and 259, depending upon whether the error voltage and the supply voltage are in or out of phase, and, ultimately, whether P2 is too high or too low.

FIGURE 15 is a schematic diagram of an alternative arrangement for measuring the pressure differential DP between chambers 52 and 54. Four air capacitors 264, 266, 268 and 270 are connected together to form a capacitance bridge. Capacitors 268 and 270 are located, respectively, within chambers 52 and 54. The air in chambers 52 and 54 provides the dielectric for these capacitors. A variable capacitor 272 is connected in parallel with air capacitor 268. As the pressure is varied in master chamber 52, and in slave chamber 54, the pressure of the air between the plates of capacitors 268 and 270 varies, thus varying the dielectric constant of this air, and, hence, the capacitance of each of these capacitors. Variable capacitor 272 is used to balance the bridge in the presence of a pressure differential DP.

A high-frequency A.C. source 274 energizes the bridge through an isolating transformer 276. An amplifier 278 is connected to the null point of the bridge. If the pressure P differs from the pressure P by more or less than DP, the bridge will become unbalanced and an error signal will be sent to amplifier 278. This amplified error signal is transmitted to a phase-sensitive demodulator 280. The output of demodulator 280 can be used in the same manner as the output of differential transformer. 190 to control pressure scanning apparatus. An advantage of this alternative pressure arrangement is its simplicity and its compactness.

Another alternative pressure measurement arrangement is shown in FIGURE 16. It is the same as that shown in FIGURE 15 except that capacitors 264 and 266 are 10- 15 cated in chamber 54 together with capacitor arrangement also is simple and compact.

The above description of the invention is intended to be illustrative and not limitnig. Various changes or modifications in the embodiment described may occur to those skilled in the art and these can be made without departing from the spirit or scope of the invention as set forth in the claims.

We claim:

1. An interferometric spectrometer comprising, in combination, at least three Fabry-Perot etalons optically aligned in series with one another, each of said etalons having a pair of optical plates with a predetermined plate spacing between them, the plate spacing of each of said etalons being different from the plate spacing of each other of said etalons, and means for directing light into said etalon plates, said etalons being aligned in series with beam, and directing said beam along the central axes of said etalon plates, sai detalons being aligned in series with non-focusing light transmitting means intermediate said etalons, the ratios of the plate spacing being chosen so that one peak of the Airy function curve of one of said etalons coincides in frequency with one peak of the airy function curve of each other etalon, and all other peaks of said Airy functions are substantially non-coincident in frequency.

2. Apparatus as in claim 1 including means for scanning said spectrometer, and in which the minimum response of said spectrometer to the absorption of the resonance line D of sodium vaipor held at a temperature of around 180 C. is no more than approximately percent greater than its minimum response to a black card source.

3. Apparatus as in claim 1 including means for housing said etalons and said light-directing means in a linear array, and means for supporting said array and housing means in a substantially vertical orientation.

4. Apparatus as in claim 1 including means for varying the index of refraction of the gas between the plates of said etalons, said index-varying means being adapted to maintain a constant, direct functional relationship between the index of refraction of the gas in each of said etalons.

5. Apparatus as in claim 3 in which said housing means comprises a plurality of housings each enclosing one of said etalons, each of said housings having a first opening in a first side-wall and another opening in a second wall opposite to said first side-wall, a first transparent windowpane fitted into said first opening, a mounting member fitted into said second opening, a second transparent windowpane secured to said mounting member so as to seal said second opening, said etalon being mounted and secured to said mounting member.

6. Apparatus as in claim 5 in which one of said windowpanes is aligned generally transversely to the plates of the etalon mounted in the housing.

7. Apparatus as in claim 5 including an airtight cover for said housing, said cover including a plate, a handle rotatably mounted on said plate, a bar secured to said handle and extending beyond the edge of said plate, a pair of notched members secured to said housing upon opposite ones of its sides, said notched members being aligned with their notches positioned to receive the ends of said bar when said handle is rotated, and being adapted to wedge said bar tightly between said plate and said members to press said plate tightly against the edges of said housing when said handle is turned to move said bar into engagement with said notches.

8. In an interferometric spectrometer utilizing a plurality of Fabry-Perot etalons optically aligned in series with one another, each of said etalons having a pair of optical plates with a predetermined plate spacing between them, means for directing light into said etalons, and means for detecting and recording the variation of light intensity emitted by said etalons, means for varying the 270. This frequency of light waves transmitted by said etalons and thereby scanning said spectrometer, a gas-tight chamber enclosing each of said etalons, rneans for varying the pressure of the gas in each of said chambers, said pressurevarying means including a source of high-pressure gas, means for connectingsaid chambers to said high-pressure source and metering the flow of gas from said source into each of said chambers, means for measuring the difference in pressure between first and second ones of said chambers, said pressure-measuring means being adapted to generate an error signal corresponding to the deviation in the pressure of said second chamber from a value which is different from the pressure of said first chamber by a pre-set differential amount, and :means responsive to said error signal for adjusting the pressure in said second chamber and maintaining constant the amount of said pressure differential.

9. Apparatus as in claim 8 in which said error-signalresponsive means is adapted to adjust the rate at which said metering means allows gas to flow into said second chamber to correspond to the rate at which gas flows into said first chamber in order to equalize said rates.

10. Apparatus as in claim 9 in which said metering means includes a first metering valve controlling the fiow of gas into said first chamber and a second metering valve controlling the flow of gas into said second chamber, and in which said error-signal-responsive means includes a sink having relatively low gas pressure and means for by-passing the fiow of gas from said high-pressure source around said second metering valve and into said second chamber, said error-signal-responsive means being adapted to selectively connect said second chamber to either said low-pressure source or said by-pass means in order to adjust said pressure in said second chamber to a desired value. p

11. Apparatus as in claim 10 in which said second metering valve includes control apparatus responsive to gas pressure for adjusting the metering rate of said second valve, said error-signal-responsive means also being adapted to selectively connect said control apparatus of said second metering valve to either said high-pressure or said low-pressure source in order to' adjust the metering rate of said second valve.

12. Apparatus as in claim 11 in which said error-signalgenerating means is adapted to generate an electrical signal and said means for selectively connecting said second chamber and said control apparatus for said second metering valve to either said low-pressure or said high-pressure source includes solenoid valves adapted to be operated by said electrical error signal.

13. Apparatus as in claim 12 in which said pressuremeasurement means includes a mercury manometer connected between said first and second chambers, a ferrite sphere floating upon the surface of the column of mercury in said manometer tube, and a differential transformer associated with said ferrite sphere for developing an error signal whose magnitude and phase is a function of the position of said ferrite sphere with respect to said differential transformer.

14. Apparatus as in claim 8 which is adapted to maintain the pressure in each of said first and second chambers constant while maintaining constant the difference between their pressures, or to increase the pressures in both chambers while maintaining the difference between their pressures constant, or to reduce the pressure in said chambers while maintaining said pressure differential at a constant value. p

15. Apparatus as in claim 11 in which said pressurevarying means is adapted to evacuate said chambers while maintaining constant the pressure differential between said chambers and simultaneously maintaining constant the value of pressure within said control apparatus for said second metering valve.

(References on following page) 1 7 References Cited UNITED STATES PATENTS 6/1962 Dobrowolski 350-166 5/1964 Kisliuk et a1. 33194.5

OTHER REFERENCES l8 eter, Journal of the Optical Society of America, v01. 41, No. 5, May 1951, pp. 351353.

Biondi: High-Speed, Direct Recording Fabry-Perot Interferometer, The Review of Scientific Instruments, v01. 27, No. 1, January 1956, pp. 36-39.

DAVID H. RUBIN, Primary Examiner.

JOHN K. CORBIN, JEWELL H. PEDERSEN,

Examiners.

R. I. STERN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,373,651 March 19, 1968 Julian E. Mack et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 15, line 16, after "into" insert confining said light into a relatively thin beam, and directing said beam along the central axes of lines 18 and 19, strike out "beam, and directing said beam along the central axes of said etalon plates, sai detalons being aligned in series with".

Signed and sealed this 24th day of June 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, IR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

